This invention pertains to microlithography (projection-transfer of a pattern, defined on a reticle or mask, to a xe2x80x9csensitivexe2x80x9d substrate using an energy beam). Microlithography is a key technology employed in the fabrication of microelectronic devices such as integrated circuits, displays, magnetic pickup heads, and micromachines. More specifically, the invention pertains to microlithography performed using a charged particle beam (e.g., electron beam or ion beam) as the energy beam. Even more specifically, the invention pertains to altering the focusing conditions to realize with good efficiency the correction of shape defects in transfer images caused by differences in the pattern distribution within respective subfields and manufacturing error at the time of mask preparation, and thus to realize high-resolution, high-precision pattern transfer and exposure, in a charged-particle-beam exposure method using a so-called divided-reticle transfer system.
The prior art is discussed in the context of using an electron beam to perform microlithography of a complex pattern, such as for a layer of an integrated circuit, onto the surface of a suitable substrate such as a semiconductor wafer. Electron-beam microlithography offers prospects of very high accuracy and pattern resolution, but to date the throughput realized using electron-beam microlithography has been disappointingly low. Various approaches have been investigated to solve the low-throughput problem.
Example approaches currently being utilized to a limited extent include xe2x80x9ccell projection,xe2x80x9d xe2x80x9ccharacter projection,xe2x80x9d and xe2x80x9cblock exposurexe2x80x9d (collectively termed xe2x80x9cpartial-blockxe2x80x9d pattern transfer). Partial-block pattern transfer is used especially whenever the pattern to be transferred to the substrate comprises a region in which a basic pattern unit (or several basic pattern units) is repeated many times. For example, partial-block pattern transfer generally is used for patterns having large memory circuits, such as DRAMs. In such patterns, the basic pattern units are very small, having measurements on the substrate of, for example (5 xcexcm)2 (i.e., 5 xcexcmxc3x975 xcexcm). The basic pattern units are defined on one or several exposure units on the reticle and the exposure units are exposed repeatedly many times onto the substrate to form the pattern on the substrate. Unfortunately, partial-block pattern transfer tends to be employed only for repeated portions of the pattern. Portions of the pattern that are not repeated are transferred onto the substrate using a different method, such as the variable-shaped beam method. Therefore, partial-block pattern transfer has a throughput that is too low, especially for large-scale production of integrated circuits and other microelectronic devices.
Another approach is electron-beam xe2x80x9cdirect writingxe2x80x9d in which the pattern is drawn on the substrate line by line. Whereas this approach has application in preparing reticles and masks, the throughput obtained with this technique is much too low to be practical for large-scale production of integrated circuits and other microelectronic devices. During such direct writing, the shape of the beam can be changed (termed xe2x80x9cvariable-shaped beamxe2x80x9d pattern transfer).
Yet another conventional approach that has been investigated in an effort to achieve a higher throughput than partial-block pattern-transfer methods is a projection microlithography method in which the entire reticle pattern for a complete die (or even multiple dies) is projection-exposed onto the substrate in a single xe2x80x9cshot.xe2x80x9d In such a method, the reticle defines a complete pattern, such as for a particular layer in an entire integrated circuit. The image of the reticle pattern as formed on the substrate is xe2x80x9cdemagnifiedxe2x80x9d by which is meant that the image is smaller than the pattern on the reticle by a xe2x80x9cdemagnification factorxe2x80x9d (typically 4:1 or 5:1). To form the image on the substrate, a projection lens is situated between the reticle and the substrate. Whereas this approach offers prospects of excellent throughput, it to date has exhibited excessive aberrations and the like, especially of peripheral regions of the projected pattern. In addition, it is extremely difficult to manufacture a reticle defining an entire pattern that can be exposed in one shot.
Yet another approach that is receiving much current attention is the xe2x80x9cdivided-reticlexe2x80x9d projection-exposure approach that utilizes a xe2x80x9cdivided,xe2x80x9d xe2x80x9cpartitioned,xe2x80x9d or xe2x80x9csegmentedxe2x80x9d reticle. On such a reticle, the overall reticle pattern is subdivided into portions termed herein xe2x80x9cexposure units.xe2x80x9d The exposure units can be of any of various types termed xe2x80x9csubfields,xe2x80x9d xe2x80x9cstripes,xe2x80x9d etc., as known in the art. For exposure, the reticle is mounted on a reticle stage and the substrate is mounted on a substrate stage. Each exposure unit is exposed individually and sequentially in a separate xe2x80x9cshotxe2x80x9d or scan. The image of each exposure unit is projection-exposed (typically at a demagnification ratio such as 4:1 or 5:1) using a projection-optical system situated between the reticle and the substrate. Even though the projection-optical system typically has a large field, distortions, focal-point errors, and other aberrations, and other faults in projected images of the exposure units are generally well-controlled. Although divided-reticle projection-exposure systems provide lower throughput than systems that expose the entire reticle in one shot, divided-reticle projection-exposure systems exhibit better exposure accuracy and image resolution over the entire pattern as projected.
Further regarding divided-reticle projection-exposure systems, exposure of each exposure unit generally is performed with the focal point of the projected image on the surface of the substrate. Also, as each exposure unit is projected onto the substrate, aberrations such as distortion are corrected. The respective images of the exposure units are formed at corresponding locations on the substrate such that the images are xe2x80x9cstitchedxe2x80x9d together in a contiguous manner. Such stitching usually is performed by a combination of stage movements and beam deflection.
In divided-reticle projection-transfer using a charged particle beam, despite the control that conventionally can be exerted during projection of each exposure unit, certain problems nevertheless can arise. For example, if the beam current of the charged particle beam used to form the image is excessively large, then imaging can be affected adversely due to mutual repulsion of the charged particles in the beam. This phenomenon is termed the xe2x80x9cCoulomb effect.xe2x80x9d Also, the quality of imaging from one exposure unit to the next can be inconsistent, based upon changes in xe2x80x9cfeature densityxe2x80x9d or xe2x80x9cfeature distributionxe2x80x9d from one exposure unit to another. Conventionally, changes can be made in real time to any of various imaging parameters to correct most of these changes. To such end, a modern charged-particle-beam (CPB) microlithography apparatus has a complex system for making subtle corrections to the optical performance of the system as exposure progresses. For example, in a modem variable-shaped beam system, focus correction can be predicted from the transverse area of the shaped beam and from other apparatus parameters such as acceleration voltage, beam-current density, beam-divergence angle, and axial length of the CPB-optical system.
In a conventional divided-reticle CPB-microlithography system, the dimensions of each exposure unit on the reticle range from approximately 10-xcexcm square to 1000-xcexcm square. (This area is extremely large compared to the area exposed per shot in any of the partial-block or variable-shaped beam approaches.) It has been reported that, in cases in which the area of the image is in this range, variations in imaging properties due to the Coulomb effect is small. See, Berger et al., xe2x80x9cParticle-particle Interaction in Image Projection Lithography,xe2x80x9d J Vac. Sci. Technol. B11(6):2294, November/December 1993. According to conventional thinking, this allows the upper limit of beam current (at which the magnitude of variations in image properties can be kept below a specified threshold) to be increased so as to increase throughput.
In general, a reticle pattern is not distributed uniformly over each exposure unit. Rather, especially for complex patterns, the pattern is distributed over the reticle in a non-uniform manner. As a result, the manner in which image properties vary differs according to the pattern distribution. Such variations in image properties include variations in astigmatism, image focus, image rotation, image magnification, and image position for each exposure unit.
Also, errors arising at the time of reticle manufacture (e.g., errors arising during electron-beam drawing of the reticle and during subsequent etching of the pattern into the reticle substrate) cannot be predicted and corrected at the time the reticle pattern is designed. For example, errors can arise from deviations from specifications occurring during manufacture of the reticle from the pattern-design data. This would not be a problem if ideal manufacture of reticles were possible. However, since reticle manufacture remains an imperfect craft, corrections are required to correct or compensate for such errors, especially if maximal resolution must be achieved of the pattern as projected.
In view of the shortcomings of the prior art as summarized above, an object of the invention is to provide charged-particle-beam (CPB) microlithography apparatus and methods in which reticle-pattern manufacturing errors within subfields (or specified groups of subfields) can be corrected efficiently during exposure of the subfields or groups of subfields. Hence, high-resolution, high-accuracy lithographic pattern transfer and exposure can be realized.
To such ends, and according to a first aspect of the invention, CPB microlithography apparatus are provided for transferring a pattern, defined on a segmented reticle in which the pattern is divided into multiple subfields each defining a respective portion of the pattern, to a sensitive substrate. An embodiment of such an apparatus comprises a substrate stage, a reticle stage, an illumination-optical system, a projection-optical system, and a main controller. The substrate stage is configured to hold the substrate. The reticle stage is situated upstream of the substrate stage and configured to hold the reticle. The illumination-optical system is situated upstream of the reticle stage and configured to illuminate groups of subfields successively on the reticle using a charged-particle illumination beam, wherein each group consists of at least one respective subfield. The projection-optical system is situated between the reticle stage and the substrate stage. The projection-optical system is configured to direct an imaging beam, formed from particles of the illumination beam passing through the reticle, to the substrate such that respective images of the illuminated subfields are formed at respective positions on the substrate serving to stitch the images together. The main controller is connected to the reticle stage, substrate stage, illumination-optical system, and projection-optical system. The main controller is configured to command one or both the illumination-optical system and the projection-optical system to perform a respective optical correction as each subfield is being exposed. The optical correction is based on respective reticle-pattern-inspection data for each of the subfields. With respect to this embodiment, the optical correction can be, for example, one or more of shape-astigmatic aberration, focusing-astigmatic aberration, image focal point, image rotation, image magnification, and image position on the substrate. The optical correction can be made according to one or more respective correction values. The correction values can be determined by actual measurement data of the pattern as defined on the reticle and/or by optical simulation. The correction values can include one or more apparatus constants such as beam-acceleration voltage, beam-current density, beam-divergence angle, and optical-system length.
Another apparatus embodiment comprises the substrate stage, reticle stage, illumination-optical system, and projection-optical system as summarized above. This embodiment also includes a main controller connected to the reticle stage, substrate stage, illumination-optical system, and projection-optical system. The main controller comprises a memory in which are stored index data for various subfields based on respective reticle-pattern-inspection data for the subfields. Also stored in the memory are optical-correction data for the various subfields corresponding to the reticle-pattern-inspection data. The index data and corresponding optical-correction data desirably are stored as a look-up table that is consulted as each of the various subfields is being exposed so that exposure of each of the various subfields is corrected optically according to the recalled respective optical-correction data.
According to another aspect of the invention, methods are provided for performing CPB microlithography of a pattern to a sensitive substrate. In an embodiment of such a method, the pattern is defined on a segmented reticle in which the pattern is divided into multiple subfields each defining a respective portion of the pattern. Groups of subfields on the reticle (each group consisting of at least one respective subfield) are illuminated successively using a charged-particle illumination beam. As each group of subfields is illuminated, an imaging beam, formed from particles of the illumination beam passing through the reticle, is directed to the substrate such that respective images of the illuminated subfields are formed at respective positions on the substrate serving to stitch the images together. As each subfield is exposed, a respective optical correction is performed that is based on respective reticle-pattern-inspection data for each of the subfields. In this method, the optical correction can be one or more of shape-astigmatic aberration, focusing-astigmatic aberration, image focal point, image rotation, image magnification, and image position on the substrate. The optical correction can be made according to one or more respective correction values. The correction values can be determined by actual measurement data of the pattern as defined on the reticle and/or by optical simulation. The correction values can include one or more apparatus constants such as beam-acceleration voltage, beam-current density, beam-divergence angle, and optical-system length. The index data can be obtained from a previously performed reticle inspection made at the time of reticle fabrication. The index data can include one or more of image rotation and pattern-element positions within the respective subfields.
In another method embodiment, the pattern is defined on a segmented reticle as summarized above. Index data for various subfields are stored, based on respective reticle-pattern-inspection data for the subfields. Also stored are optical-correction data for the various subfields corresponding to the reticle-pattern-inspection data. These data desirably are stored as a look-up table. Groups of subfields on the reticle are illuminated successively using a charged-particle illumination beam, wherein each group consists of at least one respective subfield. As each group of subfields is illuminated, an imaging beam (formed from particles of the illumination beam passing through the reticle) is directed to the substrate such that respective images of the illuminated subfields are formed at respective positions on the substrate serving to stitch the images together. Also, as each of the various subfields is being exposed, the look-up table is consulted to obtain respective optical-correction data for the subfield. The optical-correction data are applied to optically correct exposure of each of the various subfields.
To improve the efficiency with which the correction values are obtained, correction indices (e.g., rankings or the like) can be determined from the pattern-element data within the respective subfields. Only these indices need be supplied as exposure data. At the same time, correction amounts corresponding to the indices are stored in the look-up table. During exposure of the respective subfields, correction amounts corresponding to the indices are recalled from the table, and corrections are performed in accordance with the data. The correction amounts in the table can be rewritten as desired, such as in accordance with changes in, e.g., beam-current density and beam-divergence angle. Furthermore, it is not necessary to store correction amounts for all correction ranks; intermediate values can be obtained by interpolation, thereby reducing the size of the table.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.